Chemielectron and chemiion spectra have been recorded for the reaction of oxygen atoms with ethyne, C2H2 Four chemielectron bands have been observed with maxima at 0.06 +/- 0.04, 0.10 +/- 0.05, 0.23 +/- 0.04, and 0.50 +/- 0.10 eV, whose relative intensities depend upon the relative partial pressure of the reactants used. The chemiion spectra show HCO+ to be a primary ion under all experimental conditions used. This evidence, as well as the behavior of the chemielectron bands with added quenching gases (N2O, CH4, and NO) and comparison of the high kinetic energy onsets of the chemielectron bands with calculated reaction enthalpies, allows the 0.06 and 0.23 eV bands to be assigned to the O + CH(X(2) Pi) --> HCO+ + e(-) and O + CH(a(4) Sigma(-)) --> HCO+ + e(-) reactions, respectively. The associative ionization reactions responsible for the bands at 0.10 and 0.50 eV cannot definitively be assigned, but suggestions are made on the basis of available evidence.